Mechanical properties of Sn-58Bi, In-3Ag and SAC305 solders measured with fine diameter specimens

Lead-free solders such as SnAg or SnAgCu is widely used for flip chip joining. As a result of higher melting temperature and higher elastic modulus of the lead-free solders compared to eutectic SnPb solder, defect of crack or damage in low-k dielectric layers under bond pad for flip chip joining in the semiconductor chip became a serious issue. The crack or damage is caused by mismatch of coefficient of thermal expansion (CTE) between chip and chip carrier substrate after chip join reflow. Various low melting temperature solders have been investigated and proposed as alternatives to SnAg or SnAgCu solders. Mechanical properties including creep properties are important factors as well as melting temperature for stress relief in micro joints and low-k layers. In this paper, the authors report measurement results of creep properties with fine-diameter-specimens (0.5 mm in diameter) of two low melting temperature solders (Sn-58Bi and In-3Ag) and conventional lead-free solder (Sn-3.0Ag-0.5Cu). The measurement matrix consists of four different strain rates and three different temperatures. The stress component n and the constant A in Norton's creep law were derived for each solder alloy.